Bioconversion of Renewable Resources into Ethanol: An Economic Evaluation of Selected Hydrolysis, Fermentation, and Membrane Technologies

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Abstract
Four renewable agricultural resources were considered in a process design analysis for the industrial production of ethanol. Raw materials considered were wood, molasses, whey permeate, and starch. Final fermentation substrates were diluted and/or concentrated to give equivalent sugar concentrations for each case. Renewable resource costs were expressed as $/kg of sugar rather than $/kg of the raw material. Molasses sugars were cheaper than sugars derived from the other raw materials. Various fermentation technologies were considered, including continuous culture and cell recycle. Ethanol recovery was examined using pervaporation and costs compared with distillation. The effects on ethanol prices of raw material costs, fermentation technology, product recovery, tax, plant size, and Lang factor are presented. Cultures of Candida shehatae, Zymomonas mobilis, Kluyveromyces marxianus var. Iactis and Saccharomyces cerevisiae (with Zymomonas mobilis) were used, depending on the substrate. The report identifies the most appropriate technologies in terms of final ethanol price.